Parallellize a for loop in Java using multi-threading

Question

I am very new to java and I want to parallelize a nested for loop using executor service or using any other method in java. I want to create some fixed number of threads so that CPU is not completely acquired by threads.

    for(SellerNames sellerNames : sellerDataList) {
        for(String selleName : sellerNames) {
        //getSellerAddress(sellerName)
        //parallize this task
        }
    }

size of sellerDataList = 1000 and size of sellerNames = 5000.

Now I want to create 10 threads and assign equal chunk of task to each thread equally. That is for i'th sellerDataList, first thread should get address for 500 names, second thread should get address for next 500 names and so on.
What is the best way to do this job?

Answer 1

There are two ways to make it run parallelly: Streams and Executors.

Using streams

You can use parallel streams and leave the rest to the jvm. In this case you don't have too much control over what happens when. On the other hand your code will be easy to read and maintain:

    sellerDataList.stream().forEach(sellerNames -> {
        Stream<String> stream = StreamSupport.stream(sellerNames.spliterator(), true); // true means use parallel stream
        stream.forEach(sellerName -> {
            getSellerAddress(sellerName);
        });
    });

Using an ExecutorService

Suppose, you want 5 Threads and you want to be able to wait until task completion. Then you can use a fixed thread pool with 5 threads and use Future-s so you can wait until they are done.

    final ExecutorService executor = Executors.newFixedThreadPool(5); // it's just an arbitrary number
    final List<Future<?>> futures = new ArrayList<>();
    for (SellerNames sellerNames : sellerDataList) {
        for (final String sellerName : sellerNames) {
            Future<?> future = executor.submit(() -> {
                getSellerAddress(sellerName);
            });
            futures.add(future);
        }
    }
    try {
        for (Future<?> future : futures) {
            future.get(); // do anything you need, e.g. isDone(), ...
        }
    } catch (InterruptedException | ExecutionException e) {
        e.printStackTrace();
    }